The present technology generally relates to coating systems and methods suitable for protecting components exposed to high-temperature environments, such as the hostile thermal environment of a turbine engine. More particularly, this technology is directed to an Environmental Barrier Coating (EBC) on a silicon-containing region of a component and to the incorporation of surface features in the silicon-containing region to inhibit creep displacement of the EBC when subjected to shear loading at elevated temperatures.
Higher operating temperatures for turbine engines are continuously sought in order to increase their efficiency. Though significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, alternative materials have been investigated. Ceramic composite materials are currently being considered for such high temperature applications as combustor liners, vanes, shrouds, blades, and other hot section components of turbine engines. Some examples of ceramic composite materials include silicon-based composites, for example, composite materials in which silicon, silicon carbide (SiC), silicon nitride (Si3N4), and/or a silicide serves as a reinforcement phase and/or a matrix phase.
In many high temperature applications, a protective coating is beneficial or required for a Si-containing material. Such coatings should provide environmental protection by inhibiting the major mechanism for degradation of Si-containing materials in a water-containing environment, namely, the formation of volatile silicon hydroxide (for example, Si(OH)4) products. A coating system having these functions will be referred to below as an environmental barrier coating (EBC) system. Desirable properties for the coating material include a coefficient of thermal expansion (CTE) compatible with the Si-containing substrate material, low permeability for oxidants, low thermal conductivity, stability and chemical compatibility with the Si-containing material.
The silicon content of a silicon-containing bondcoat reacts with oxygen at high temperatures to form predominantly an amorphous silica (SiO2) scale, though a fraction of the oxide product may be crystalline silica or oxides of other constituents of the bondcoat and/or EBC. The amorphous silica product exhibits low oxygen permeability. As a result, along with the silicon-containing bondcoat, the silica product that thermally grows on the bondcoat is able to form a protective barrier layer.
The amorphous silica product that forms on a silicon-containing bondcoat in service has a relatively low viscosity and consequently a high creep rate under shear loading. High shear loads (e.g. from about 0.1 to 10 MPa) can be imposed by g forces (e.g. from about 10,000 to about 100,000 g's) resulting from high-frequency rotation of moving parts, such as blades (buckets) of turbine engines. Such shear loading may cause creep displacements of the EBC relative to the bondcoat and substrate which can result in severe EBC damage and loss of EBC protection of the underlying substrate.